Transistors, such as metal oxide semiconductor field-effect transistors (MOSFETs), are the core building block of the vast majority of semiconductor devices. Some semiconductor devices, such as high performance processor devices, can include millions of transistors. For such devices, decreasing transistors size, and thus increasing transistor density, has traditionally been a high priority in the semiconductor manufacturing industry.
As device geometries shrink, the gate leakage current contributes proportionally more to the overall device leakage current. One technique to reduce the gate leakage current is to replace the gate insulator material with a dielectric material having a higher dielectric constant (e.g., a high-k dielectric), such as a hafnium-based oxide. However, the threshold voltage (Vt) of a high-k device is more sensitive to the device width (W). Because device width will vary from one device to another depending on the design needs, when high-k dielectric materials are used in conventional processes, the variation in the gate widths produces unacceptably wide variations in threshold voltages among devices.
In practice, the width of the gate stack is often greater than the width of the underlying semiconductor material and overlaps onto the field oxide. This accounts for potential misalignment as well as rounding of an end of the gate line and ensures that the length of the channel formed during subsequent ion implantation steps is consistent along the device width and that the drain and source regions of the device are not contiguous. However, it is believed that the threshold voltage sensitivity in high-k devices is attributable to oxygen diffusing from the field oxide to the gate stack, also known as the antenna effect. One proposed approach to this issue involves nitridation of the surface of the field oxide. While this reduces the threshold voltage sensitivity, the threshold voltage remains sensitive to device width because oxygen from the field oxide underlying the nitride will still diffuse to the gate stack.